1. Field of the Invention
The present invention relates to a pressure sensor for detecting a pressure level with the use of semiconductor strain gauge.
2. Description of the Prior Art
According to the basic principle of the pressure sensor, an impurity is diffused into a single crystal of semiconductor such as silicon to prepare a gauge resistor so that the change in the resistance of the gauge resistor as a result of the piezoelectric effect due to a strain is converted into an electric signal and is measured.
The pressure sensor is roughly divided into two type, i.e., the absolute pressure type and the differential pressure type in respect to its function. The pressure sensor according to the present invention is a kind of the so-called "absolute pressure type", in which an external pressure is detected with reference to the vacuum established in such a space as is defined by adhering a stationary support supporting a silicon diaphragm and a silicon bed, whereby the absolute pressure can be measured.
FIG. 1 is a sectional view showing a pressure sensor chip to be used in a pressure sensor of the absolute pressure type. In FIG. 1: numeral 11 indicates a diaphragm made of silicon; 12 a stationary support supporting the diaphragm 11; 13 a bed made of silicon; 14 a brazing material adhering the stationary support 12 and the bed 13; 15 an evacuated space which is defined by the diaphragm 11, the stationary support 12 and the bed 13; 16 a gauge resistor which is formed in the surface of the diaphragm 11 by the impurity diffusion process; and 17 resistors which are formed on the stationary support 12. These components thus far described constitute a pressure sensor chip 10. The diaphragm 11 is warped toward the bed 13 by the external pressure.
FIG. 2 is a sectional view showing the pressure sensor of the absolute pressure type according to the prior art, in which the pressure sensor chip is directly adhered to a package. In FIG. 2: numeral 10 indicates the pressure sensor chip; 20 the cap of a package; 21 the cap body of the cap 20; 22 an external pressure introduction pipe of the cap 20; 30 a stem; 31 the metal frame of the step 30; 32 the external leads of the stem 30; 33 the insulator of the stem 30; 40 the brazing material adhering the pressure sensor chip 10 to the metal frame 31 of the stem 30; and 50 internal wirings such as thin gold wires connecting the electrodes 17 of the pressure sensor chip 10 and the external leads 32. The cap 20 and the stem 30 constitute together the package.
If the pressure sensor chip 10 is directly attached by means of the brazing material 40 or an adhesive to the package stem 30, as in the above, a strain is applied to the diaphragm 11 as a result of the difference in thermal expansion between silicon and the material of the package. The stress resulting from that strain is detected by the pressure sensor itself, and the correction of the detected value has been required for practical uses. Nevertheless, it has been difficult to accurately control the stress resulting from the strain, and the correction has been possible for experimental purposes but hardly impossible for mass-production.
FIG. 3 is a sectional view showing a conventional pressure sensor which has been conceived to reduce the thermal strain resulting from the difference in thermal expansion. In FIG. 3, reference numerals similar to those appearing in FIG. 2 indicate parts similar to those of FIG. 2. Numeral 60 indicates a silicon support having an adhesion area of 1 to 4 mm.sup.2, which is attached to the opposite side of the pressure sensor chip 10 to the diaphragm 11 and which is adhered to the metal frame 31 of the stem 30 by means of an eutectic brazing material of Au-Si, for example. The pressure sensor shown in FIG. 3 is so devised that the thermal strain due to adhesion between different materials may hardly be transmitted to the pressure sensor chip 10 by sandwiching the silicon support 60 having the small adhesion area between the pressure sensor chip 10 and the stem 30. However, this device has not succeeded in settling the subject matter drastically, but there has been no way of avoiding that more or less thermal strain is transmitted to the diaphragm 11.
It is obvious that the thermal strain resulting from the adhesion between the package and the bed can be solved by completely floating the pressure sensor chip over the package. The reason why that simple fact has never been put into practice is to make remarkably difficult the subsequent wire bonding of the internal wirings. Even if the wire bonding could be performed, the support of the pressure sensor chip exclusively by the internal wirings would invite a danger that the pressure sensor chip is shifted to break the internal wirings, as has been accepted in the art, thus raising a practical problem.